The present invention relates to a drive system for a vehicle driveline comprising a pair of interconnected first transverse drive shafts extending in opposite directions, each of which supports a ground engaging member, and an arrangement allowing said pair of first transverse drive shafts to rotate at different speeds. The invention also relates to an axle comprising said pair of transverse drive shafts for a vehicle driveline and a vehicle comprising the driveline.
The invention will below be described for a frame-steered work vehicle in the form of an articulated hauler. This should however be regarded as a non-limiting example. The invention may also be applied in other frame-steered work vehicles, like a wheel loader, backhoe loader, tractor and other types of work vehicles, like an excavator or a skid steer vehicle. The term “ground engaging member” is used for drive wheels or caterpillar tracks. The invention may further be applied in a passenger car, truck or bus.
In addition to delivering power from a source of motive power to a number of drive wheels, the drive system must permit the respectively driven wheels to rotate at different speeds. In a turn, for example, the outside wheels must rotate faster than the inside wheels, and the front wheels must rotate faster than the rear wheels. Typically, the pair of first transverse drive shafts rotatively driving a right and left drive wheel, respectively are interconnected by a connection arrangement in the form of a differential mechanism which permits the opposite drive wheels to rotate at different speeds while delivering an approximately equal amount of drive torque to each. A longitudinal drive shaft rotatively connects a vehicle gear box with the differential mechanism. However, this characteristic of delivering approximately equal amounts of torque to the drive wheels independently of their relative rotational speed significantly limits the total amount of torque which can be delivered to the drive wheels when one of the drive wheels loses traction.
One way of overcoming this problem is to use a clutch mechanism that resists any differential rotation between the drive wheels. In other words, unequal torque distributions between drive wheels are supported by sacrificing some of the differential capacity to support unequal rotational speeds between the drive wheels. Thus, the clutch locks the differential mechanism. Such a clutch is normally, at least for work vehicles, formed by an on-off claw clutch.
In a drive system for all wheel drive, power is delivered to different combinations of drive wheels. In addition to delivering power to both a front and at least one rear drive axle, all wheel drives must also permit the two axles to rotate at different speeds. Accordingly, the longitudinal drive shafts to the front and rear axle are often interconnected by a differential mechanism which permits the front and rear drive axles to rotate at different speeds while delivering approximately equal amounts of torque to them. It is also known to use a clutch mechanism for locking such longitudinal differential.
A frame-steered work vehicle, such as an articulated hauler, comprises a forward vehicle section comprising a forward frame and a front wheel axle supported by the frame and a rear vehicle section comprising a rear frame and at least one rear wheel axle supported by the frame. The forward frame is connected to the rear frame by means of an articulation joint allowing the forward vehicle section and the rear vehicle section to pivot relative to one another about a vertical axis. A pair of hydraulic cylinders is arranged one on each side of the articulation joint and controlled by the driver via a steering wheel and/or a joy stick for steering the vehicle.
In an articulated hauler the forward vehicle section comprises an engine for propelling the vehicle and a cab. The rear vehicle section comprises a tiltable load-carrying platform. Further, the rear vehicle section has two rear wheel axles, a forward bogie axle and a rear bogie axle. Articulated haulers are used for transporting heavy loads, such as earth, gravel and stones both in areas where there are no roads, for example in connection with road and tunnel building, in sand pits, mines and similar environments, i.e. on uneven ground, and on the road. The articulated hauler is commonly designed for switching between operations using various driving wheel combinations, all six wheels on the three axles being selectively operable as driving wheels, depending on the prevailing operating conditions.
It is desirable to achieve a drive system and an axle for a vehicle driveline, which improves the control of the driveline with regard to delivery of drive torque to the ground engaging members.
According to an aspect of the present invention, the drive system comprises a pair of individually selectively engagable clutches, one clutch for each transverse drive shaft, and each of said clutches is configured for engaging and disengaging, respectively, the associated transverse drive shaft to a driving connection with a source of motive power. In such a drive system, there is no need for a differential mechanism. Instead, when one of the clutches is engaged, the transverse drive shafts are free from being in a rotational connection to each other. Thus, each ground engaging member may be controlled individually.
Preferably, said clutch is gradually engagable so that it permits, in an active state, different relative rotational speeds between an input member and an output member of the clutch. Thus, each clutch is configured to permit different degrees of slip. This creates conditions for a dynamic control of the drive line.
Further, the drive system advantageously comprises a main gear box, a first drive shaft configured to extend in a vehicle longitudinal direction and which rotatively connects the main gear box and said first pair of transverse drive shafts. Thanks to said pair of individually selectively engagable clutches, there is no need for a clutch in the main gear box.
According to a preferred embodiment, said arrangement for allowing different shaft speeds comprises a transmission comprising a pinion wheel adapted to be rotationally fixed to an associated longitudinal drive shaft and a crown wheel intermeshed with the pinion wheel and rotationally fixed to an input member of said clutch. Thus, each ground engaging member is free from being in a rotational connection to the crown wheel when the associated clutch is disengaged.
According to a further preferred embodiment, the drive system comprises a control unit operationally connected to each clutch for controlling engagement thereof. Further, the drive system comprises a plurality of rotational speed sensors for sensing the difference in rotational speeds across each clutch, and that said sensors are operatively connected to said control unit. Said control unit comprises a memory and a processor with software for controlling the engagement of the clutches. Thus, the sensed rotational speeds of the ground engaging members are received by the control unit, which determines which clutch(es) that should be engaged and to what extent and thereafter actuates the clutch(es) accordingly. Such a drive system would give an intelligent control with maximum freedom with regard to drive torques and restraint torque in the driveline.
Further, for an optimum control preferably all wheel axles of the vehicle driveline are provided with such a pair of individually selectively engagable clutches. In an articulated hauler, a transfer gear box is operationally connected between the main gear box and the longitudinal drive shafts. The purpose of the transfer gearbox is to allow shifting between a low-gear register (low) and a high-gear register (high) and also to distribute the force between the front axle and the rear axles. Thanks to said pair of individually selectively engagable clutches, there is no need for either a clutch or a differential mechanism in the transfer gear box.
According to a preferred embodiment, the drive system comprises a longitudinal clutch arranged for engaging and disengaging, respectively, the second (and third) wheel axle from a driving connection with the main gear box. By disconnecting said longitudinal clutch, only the first (front) wheel axle is in driving connection with the engine. Thus, the wheels of the second (and third) axle would rotate, however not the central transmission in the axle. This would give an energy-efficient driving, especially advantageous for transporting material longer stretches.
A further purpose of the invention is to achieve a method for controlling the power delivered to a plurality of ground engaging members of a vehicle driveline, which improves the control of the driveline with regard to delivery of drive torque to the ground engaging members.
This purpose is achieved with a method according to the following claim 27. Further advantageous embodiments and further advantages of the invention emerge from the detailed description below and the claims.